sd-bus: it's not a user error to query the error contained in a bus message

It's an OK way to check whether a message contains an erro, let's not
consider this a loggable assertion event.

src/libsystemd/sd-bus/bus-message.c

@@ -1023,7 +1023,9 @@ _public_ const char *sd_bus_message_get_sender(sd_bus_message *m) {
 
 _public_ const sd_bus_error *sd_bus_message_get_error(sd_bus_message *m) {
         assert_return(m, NULL);
-        assert_return(sd_bus_error_is_set(&m->error), NULL);
+
+        if (!sd_bus_error_is_set(&m->error))
+                return NULL;
 
         return &m->error;
 }